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Here's the latest hot debate among Fourth Amendment scholars: when magistrate judges issue search warrants on computers, should those warrants limit where in the machine's directory system the police may look, or for how long they can scan the drive?

Professor Orin Kerr of George Washington Law School says no (PDF). Assistant Professor Paul Ohm of the University of Colorado Law School says yes (PDF).

As personal information is increasingly stored on digital devices like computers and smartphones, the issue becomes hugely important, and argument about it often relies on metaphors. In the case of the net neutrality debates, comparisons were always made to the highway system; when it comes to computer searches, it's all about the filing cabinet.

Just like file cabinets?

Some courts and legal scholars have characterized computer drives as analogous to offices or warehouses, with their cabinets full of business files. Just as judges grant the cops warrants to enter those offices and rummage through files that aren't relevant to a case in order to find those that are, these scholars argue that police should be able to search through hard drives full of private data not relevant to the suspected crime.

But other courts have put the brakes on this kind of data searching, limiting what police can look for and where they can look for it. These sorts of restrictions are "unauthorized," insists Professor Kerr, because the Fourth Amendment "contemplates a narrow role for magistrate judges. Magistrate judges have no inherent power to limit how warrants are executed beyond establishing the particularity of the place to be searched and the property to be seized."

Here are some of the warrants Kerr mentions, which put limits on both data and time. Most relevant is In the Matter of the Search of: 3817 W. West End, in which a judge compelled the government to develop a specific search protocol before inspecting the computer of a tax fraud suspect.

Then there's United States v. Comprehensive Drug Testing. In that case, a magistrate judge stipulated that a warrant required forensic specialists rather than case agents to go through a computer looking for evidence of illegal steroid use.

Time-based restrictions include United States v. Brunette, a child pornography sting in which a magistrate judge imposed a time frame of 30 days for the searching. Agents seized two computers five days later. Then they applied for another 30-day search period, which they received.

Only one problem—they peeked inside the second machine shortly after the second time window closed. A district court ruled that the images found in computer number two were inadmissible because of the government's failure "to adhere to the requirements of the search warrant and subsequent order."

The hands-on approach

Professor Kerr thinks that the Supreme Court's record casts doubt on these warrant limitations. He points to Lo-Ji Sales v. New York, an "extreme case," in his words, "of a magistrate judge controlling the execution of the warrant by participating in the search," which was eventually overturned by the High Court:

Lo-Ji Sales involved a search of a bookstore for obscene materials. A police officer had purchased two obscene films from an adult bookstore, and he approached a local magistrate for a warrant to search the store. The officer wanted to seize obscene films beyond the two that he already purchased and viewed, but he did not know which of the other films satisfied the constitutional test for obscenity. He therefore asked the magistrate to help him execute the search. Under the officer’s proposal, the magistrate would accompany the officers to the store with a warrant that initially listed only the two known films as items that that the government could seize. The magistrate judge would review the additional materials himself onsite, and then tell the officers which films they could seize based on the magistrate’s judgment about what was obscene. The magistrate agreed, and he came to the search site and preapproved which films counted as obscene and therefore were seizable. After the magistrate approved the seizure of a particular film, he would add the name of the film to the warrant and the government would seize the film.

The Supremes made short work of this procedure. The judge "allowed himself to become a member, if not the leader, of the search party which was essentially a police operation," they ruled.

Kerr acknowledges that this isn't a computer case, but he also points to Dalia v. The United States. In this legal dispute the Supreme Court cast aside the objection of a defendant who complained that the police violated his Fourth Amendment rights because the wiretap warrant didn't explicitly grant them permission to enter his presence to plant a bug.

"A warrant is a judicial order allowing the government to enter the place to be searched to seize the items the judge has allowed to be seized," he says. "In contrast, how a warrant should be executed is traditionally a question for the police rather than the judge issuing the warrant."

The "preferable" way

But responding to Kerr's article, Colorado law professor Ohm finds jurisprudence suggesting that ex ante restrictions are indeed feasible. His authorities include that already mentioned Comprehensive Drug Testing case, in which the Ninth Circuit warned that a warrant "should normally include, or the issuing judicial officer should insert, a protocol for preventing agents involved in the investigation from examining or retaining any data other than that for which probable cause is shown."

And in Dalia, Ohm notes that the Supreme Court conceded that "[a]lthough explicit authorization of the entry [into an office to install a bug] is not constitutionally required, we do agree... that the 'preferable approach' would be for Government agents in the future to make explicit to the authorizing court their expectation that some form of surreptitious entry will be required to carry out the surveillance."

The bottom line for Ohm is that just letting the police ramble around or "oversearch" hard drives will allow them to survey more private data owned by innocent citizens than was ever permitted by warrants back in the days of searching warehouses full of paper. This will disrupt the balance between Fourth Amendment privacy interests and the needs of law enforcement to solve and prevent crime:

Warehouses—and even less so filing cabinets—are insignificant containers of information compared to today's hard drives, and the analogy will only become more mismatched over time. As the filing cabinet analogy fails, it takes with it the careful balance of the filing cabinet solution—haul it all away now and sort later. Today's technology poses a constitutional puzzle that is different in kind, not just in degree, from the one solved only a few decades ago.

Computers make everything different, Ohm warns. When asking for search warrants, the government must accept limits on how much digital data it can collect.

"The FBI and other law enforcement agencies are resourceful organizations full of industrious, creative, intelligent, and hard-working agents, who are dedicated to finding evidence of crime," his response concludes. "If the Fourth Amendment imposes new restrictions on what law enforcement agents can do, those agents will, as they have so many times before, find a way to continue to do their jobs efficiently and successfully while at the same time respecting the rights of the people."

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Matthew Lasar
Matt writes for Ars Technica about media/technology history, intellectual property, the FCC, or the Internet in general. He teaches United States history and politics at the University of California at Santa Cruz. Emailmatthew.lasar@arstechnica.com//Twitter@matthewlasar